Changes of the crystal structure and the lattice parameter of SrO at high pressure

The lattice parameter and the crystal structure of SrO (NaCl-type structure) have been investigated to 340 kb at 23° ± 3°C by means of X ray diffraction employing a diamond-anvil press. Silver mixed with the sample served as a pressure transmitting medium and an internal pressure calibrant. The compressional data fitted to two forms of the Birch equation yield the bulk modulus and its pressure derivative, K0 = 913 ± 27 kb, K0′ = 4.3 ± 0.3 and K0 = 906 ± 24 kb, K0′ = 4.4 ± 0.3, respectively. Between 70 and 307 kb, a splitting of the diffraction lines indicates a tetragonal distortion of the NaCl-type structure with an axial ratio of c/a = 1.005–1.008. No volume discontinuity accompanies this phenomenon, and its cause is not understood.